Since you are no longer using the cylinders, it is best to bring them back to a cylinder distributor in your area. The cylinders will not be accepted at recycling centers. Do not let these cylinders just lie around, an empty gas cylinder can still be a potetial danger. If you have proof of payment of a deposit, you can get the deposit back when returning the bottle to a re-seller of that particular company. To find a distributor in your area, you can always call the number provided on the seal of the cylinder.